1. Field of the Invention
The present invention generally relates to navigation systems, and more particularly, to receiving positioning signals at different frequency ranges, and obtaining information about a time taken to switch between the frequency ranges.
2. Description of the Related Art
Positioning systems, for example Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) and the Global Orbiting Navigational Satellite System (GLONASS), enable the location of a receiver to be accurately determined. Such systems operate by transmitting navigation messages from a plurality of sources whose locations are known. The messages include information for identifying the time at which the message was sent, which enables the receiver to determine the travel time of the signal and hence the distance to the source. This distance is referred to as a pseudorange (PR). To calculate the receiver's location in three dimensions, signals from at least four satellites are required to calculate three spatial coordinates and a clock offset between the receiver and the satellite clocks, since the receiver normally incorporates a much less accurate clock than the atomic clocks used in the satellites.
To calculate its location, the receiver receives signals from any visible satellite, i.e. any one of the satellites to which there is a direct line-of-sight for the relevant frequency range. However, in some environments, such as built-up urban areas, surrounding topographical features may block the lines of sight to many of the satellites. In this case, the minimum number of satellites to obtain a position fix may not be visible. Accordingly, it is desirable to develop a dual-mode receiver that can use satellites from more than one navigation system to calculate its location. However, in such dual-mode receivers significant errors can be introduced due to the use of different systems; for example, as a result of having to switch between the different frequencies used in the different systems.